The invention relates generally to motor vehicle brake systems. In particular the invention relates to control of a parking brake in the event of failure of a speed signal.
Modern motor vehicles have a plurality of mutually independent brake systems. A first brake system is conventionally designed as a service brake and may comprise additional systems that improve driving stability of the motor vehicle during a braking operation, for example ABS, ESP or anti-spin control. A second brake system is normally designed primarily for parking-brake operations, i.e. keeping the motor vehicle in a stationary state. For safety reasons, it is a requirement that a parking brake also has to be usable for bringing a moving motor vehicle to a standstill, for example if the service brake has failed or is functional only to a limited extent. Electrically actuable parking brake systems may moreover assist a driver when stopping or starting the motor vehicle on an ascending or descending gradient.
In order to drive an electrically actuable parking brake in different ways depending on whether the moving motor vehicle is to be braked in a “dynamic mode” of the parking brake or the stationary motor vehicle is to be kept in a stationary state in a “static mode” of the parking brake, a signal indicating a speed of the motor vehicle is processed. On the basis of this signal a driver-controlled signal to open and/or close the parking brake may then be converted in a manner appropriate to the situation into a corresponding driving of actuators of the parking brake. For safety reasons, it is a requirement that a parking brake even in the event of failure of such a speed signal still has to be usable to brake the motor vehicle and keep it in a stationary state.
For this purpose, in the European patent EP 1 610 991 B1, and corresponding U.S. Pat. No. 7,341,319 B2, both of which are incorporated by reference herein in entirety, it is proposed that an electric parking brake in the event of failure of the speed signal and activation by a driver is brought in a first step into a partially closed state. After a predetermined time has elapsed, the parking brake in a second step is then transferred suddenly from the partially closed state to a fully closed state. The proposed procedure is based on the assumption that the motor vehicle, if it was in motion during the first step, has come to a standstill before the second step.
In some situations, however, this assumption is incorrect, for example when braking from high speed or during prolonged downhill travel, and the directional stability of the vehicle may be jeopardized by the sudden full closing of the parking brake. If the parking brake of the still moving motor vehicle is namely suddenly fully closed, it is then possible for one or more of the wheels of the motor vehicle to lock, leading to an impairment of the driving stability of the motor vehicle.